Stocks which are subjected to dewaxing are usually comprised of petroleum fractions boiling above about 343.degree. C. (650.degree. F.). Thus, the molecular weight of the hydrocarbon constituents of those fractions is high, and the constituents themselves display almost all conceivable structures and structure types. The complexity of the molecular constitution of lubricating oils and its consequences are referred to in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (4th Edition), relevant portions of this text being incorporated herein by reference, for background.
Although the broad principles involved in dewaxing are qualitatively understood, the complexity of the molecular constitution of the petroleum feedstock subject to the hydrodewaxing process, presents complex considerations. That is, components of the feedstocks can undergo competing reactions, which interfere with the primary reactions sought, leading to secondary side reactions. Accordingly, the art of dewaxing has become highly developed and complex.
One of the objects of dewaxing is to render a raffinate feedstock of unacceptably high pour point to a lubricating oil base stock with a decreased, acceptable pour point. In the past, solvent dewaxing has been employed, and is a well known dewaxing process; although effective, solvent dewaxing is extremely expensive. More recently, catalytic methods for dewaxing have been proposed. Many of these current processes describe the use of molecular sieve compositions, specifically zeolites, for dewaxing.
Dewaxing of high-pour feedstocks, involves breakdown and/or rearrangement, by, for example, isomerization, of long straight chain paraffins or of long chain slightly branched paraffins, which are the wax components, sought to be removed. In addition to these wax components, other components within the feedstocks subjected to dewaxing may produce gases, and low molecular weight paraffins. Catalytic dewaxing can involve the competing reactions of breakdown of the wax component, as well as breakdown of the products of dewaxing. The latter breakdown, of products of dewaxing, can, and does, result in decrease in catalytic activity. That decrease in catalytic activity may come about by virtue of contamination of the zeolite catalyst, by low molecular weight fractions, resulting from secondary reactions, which decrease the efficiencies of the process. The production of these lower weight molecular fractions also may produce adverse effects on the quality of the ultimate product sought.
More specifically in catalytic dewaxing with molecular sieves, the lighter products compete with the heavier feed molecules for access to the cracking sites in the zeolites or silica/alumina cracking catalysts which are employed. Inasmuch as these lighter products diffuse more rapidly into the catalyst than the larger feed molecules, they have a tendency to retard the rate of conversion of the heavier molecules. Moreover, the lighter products also tend to be either more difficult to crack, such as low molecular weight paraffins, or easier to polymerize, such as low molecular weight olefins. These lower weight products also possess a tendency to coke the catalyst more readily than their heavier counterparts, thereby retarding the conversion of the heavier molecules to an even greater extent.
This competition between the light and heavy liquid petroleum molecules is rendered particularly critical when a dewaxing catalyst which includes a shape selective zeolite is used. Practically, the competitive reactions that occur during the dewaxing, catalyzed by the zeolites, results in aging of the catalyst. One result of such aging, is the necessity to increase dewaxing temperatures during dewaxing cycles. The limits of increasing the temperatures are, for practical purposes, the temperatures at which the lube products start to exhibit oxidative instability. As the hydrodewaxing temperatures approach that limit, the dewaxing unit(s) must be shut down, and the catalyst itself is then reactivated or regenerated. Depending upon the extent of reactivation, or regeneration, of the catalyst, the starting temperatures of the next cycle for dewaxing may be higher than those of the start of cycle, of the previous dewaxing cycle; thus, start of cycle temperatures may increase, after regeneration and/or reactivation of the catalyst, as well as temperature increase during the dewaxing cycle itself. These temperature increases during the dewaxing cycle, as well as the complete shutdown of the dewaxing unit during regeneration and/or reactivation of the catalyst, obviously lead to high energy inefficiencies.